Object turns white when rendering/render Layer Colors disappear
If you are using a material in the render layer's settings, it will override all other materials at the time of rendering.



To clear it press on the X to the right of the material.

Once cleared, the render layer will use the materials set for each of the objects in the scene.

EDIT:

Newer versions of blender don't use layers, but "collections" so the override function is in the scene>View Layer section.



Additional info:

Where is the Layers panel in Blender 2.8?

2.8 - Is it possible to assign (override) a material to a whole collection?